Pluto

Orbit and Distance

Farthest planet - averages 3670 million miles from the Sun (almost 6 light-hours).

More eccentric orbit than any planet. Distance varies from 2750 to 4574 million miles.

Orbits Sun in 248 years.

Can be (and now is) closer to Sun than Neptune.

Orbit tilted more (17 degrees) to the plane of the Solar System than any other planet. This
steep inclination precludes any chance of a collision with Neptune.

Period very close to 3/2 Neptune's (Resonance).

Magnitude 14 as seen from Earth; requires a sizeable telescope.

Pluto as an Ex-Planet

Despite an enormous improvement in understanding in the last two decades, what we know
of Pluto itself is still very uncertain.

Pluto is the lone planet never surveyed by spacecraft.

It was the smallest planet--at 2400 km, smaller than our Moon.

Pluto has a thin atmosphere that vaporizes at perihelion but freezes out at aphelion.

Rotates in about 6 1/3 days.

Pluto and Charon

Pluto has a large satellite (Charon)

Charon is 12,000 miles (19,400 km) away from Pluto.

Diameter roughly 800 miles (1100 km)

Period of 6 1/3 days, the same as Pluto's rotation period. (Tidal lock between Pluto and
Charon).

Much of our improved knowledge of Pluto is due to the fact that Pluto has a satellite.

Like Uranus, Pluto's pole lies nearly in the plane of its orbit and Charon's orbital plane, like
Uranus' moons, is almost perpendicular to Pluto's orbital plane.

Pluto and Charon are a double-planet system, and Charon is much nearer to its planet in size
than any other moon in the Solar System.

Pluto has a low bulk density (less than 2) and is probably an icy body much like the moons
of Saturn or Uranus.

In the late 1980's, shortly after Charon was discovered, it and Pluto underwent several years
of mutual eclipses as Earth passed through Charon's orbital plane. From these data, astronomers
got precise information on the sizes of Pluto and Charon, and also discovered:

Pluto is reddish, like Triton; Charon is neutral gray and probably made of water ice.

Pluto has a patchy surface with bright and dark patches.

Since 1992, hundreds of objects have been discovered orbiting beyond Neptune, a belt of
rock-ice asteroids (the Kuiper Belt). Once the Kuiper
Belt was discovered, many astronomers regarded
Pluto as merely the largest of this group. For a time its large size and satellite,
made it seem somewhat special, but after larger objects and objects with
satellites were discovered, it became clear Pluto's status needed
reconsideration.

In 2005, the Hubble Space Telescope discovered two other moons, which were
named Nix and Hydra. They are estimated to be 50-150 km in diameter, depending
on their albedos, and orbit 3-4 times farther away than Charon..

References

Powell, Corey S., A rare glimpse of a dim world (views of Pluto and Charon inferred from eclipse data) Scientific American, v267 p24 August 1992